Multiword information encoded by wordwise interleaving and wordwise error protection with error locative clues derived from synchronizing channel bit groups and directed to target words

ABSTRACT

A method for encoding multiword information by wordwise interleaving and wordwise error protection with error locative clues derived from synchronizing channel bit groups and directed to target words, a method for decoding such information, a device for encoding and/or decoding such information, and a carrier provided with such information. Multiword information is encoded as based on multibit symbols disposed in relative contiguity with respect to a medium. It has wordwise interleaving, wordwise error protection code facilities and error locative clues across words of a multiword group. These originate in synchronizing channel bit groups and target data words.

BACKGROUND OF THE INVENTION

The invention relates to a method as recited in the preamble of claim 1.U.S. Pat. No. 4,559,625 to Berlekamp et al and U.S. Pat. No. 5,299,208to Blaum et al disclose the decoding of interleaved and error protectedinformation, where an error pattern found in a first word may give aclue to locate errors in another word of the same group of words. Errorspointed at are relatively closer or more contiguous than other symbolsof the word that would generate the clue. The references use astandardized format and a fault model with multisymbol error burstsacross various words. Occurrence of an error in a particular word mayimply a strong probability for an error to occur in a symbol pointed atin a next word or words. The procedure will often raise the number oferrors that may be corrected before the mechanism fails.

The present inventor has recognized a problem with this method: a cluewill materialize relatively late in the process when the clueoriginating information will have been demodulated as well as been fullycorrected. This will complicate the use of higher level measures such asa retry read of the data during a later disc revolution. Also, thepresent inventor has recognized that part or all of the clues could beacquired against a lower investment in terms of the amount ofredundancy.

SUMMARY TO THE INVENTION

In consequence, amongst other things, it is an object of the presentinvention to provide a coding format with less overhead that would allowearlier generating of at least part of the clues. Now therefore,according to one of its aspects the invention is characterized accordingto the characterizing part of claim 1.

The invention also relates to a method for decoding such information, adevice for encoding and/or decoding such information, and a carrierprovided with such information. Further advantageous aspects of theinvention are recited in dependent Claims.

BRIEF DESCRIPTION OF THE DRAWING

These and further aspects and advantages of the invention will bediscussed more in detail hereinafter with reference to the disclosure ofpreferred embodiments, and in particular with reference to the appendedFigures that show:

FIG. 1, a system with encoder, carrier, and decoder;

FIGS. 2A-2C, disposition of exemplary sync patterns;

FIG. 3, a code format principle;

FIG. 4, a picket code and burst indicator subcode.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A clue or a combination of clues, once found, may cause identifying oneor more unreliable symbols. Through such identifying, such as bydefining erasure symbols, error correction will be made more powerful.Many codes will correct at most t errors when no error locations areknown. Given one or more erasure locations, often a larger number e>t oferasures may be corrected. Other types of identifying thancharacterizing as erasure symbols are feasible. Also the protectionagainst a combination of bursts and random errors may improve.Alternatively, the providing of erasure locations will for a particularfault pattern necessitate the use of only a lower number of syndromesymbols, thus simplifying the calculation. The invention may be used ina storage environment or in a transmission environment.

FIG. 1 shows a system according to the invention arranged for producingtwo types of clues, one deriving from synchronization bit groups and theother from error protected clue words, respectively. The embodiment isused for encoding, storing, and finally decoding a sequence of multibitsymbols derived from audio or video signals or from data. Terminal 20receives successive such symbols that by way of example have eight bits.Splitter 22 recurrently and cyclically transfers symbols intended forthe clue words to encoder 24, and all other symbols to encoder 26. Inencoder 24 the clue words are formed by encoding the data symbols intocode words of a first multi-symbol error correcting code. This code maybe a Reed-Solomon code, a product code, an interleaved code, or acombination thereof. In encoder 26 the target words are formed byencoding into code words of a second multi-symbol error correcting code.In the embodiment, all code words will have a uniform length, but thisis not necessary. Both codes may be Reed-Solomon codes with the firstone being a subcode of the second one. As will be shown in FIG. 4, cluewords have a higher degree of error protection.

In block 28, the code words are transferred to one or more outputs ofwhich an arbitrary number has been shown, so that the distribution on amedium to be discussed later will become uniform. Before actual writingon the medium, all code symbols are modulated into channel bits. Awell-known modulating rule adheres to a (d,k)=(1,7) constraint, thatgoverns minimum and maximum distances between consecutive signaltransitions. The modulating adapts the sequence of channel bits betterto the transmission or storage capability of the encoder-medium-decoderstring.

In this respect, FIG. 2A shows an exemplary sync pattern as stored ondisc, such as according to a pit/no pit dichotomy. The pattern as shownconsists of a sequence of nine no-pit positions followed immediately bya pattern of nine pit positions. Such pattern will violate the standardmodulating constraints if k corresponds to a sequence length of lessthan nine pit/no pit positions. For brevity, the detection signalacquired when scanning such a sequence has been ignored. The wholepattern may be bitwise inverted. The initial and final bit positions ofthe pattern may be used for other purposes, always provided thattransitions may not occur in immediately successive bit positions.

Now in FIG. 1, block 30 symbolizes the unitary medium itself such astape or disc that receives the encoded data. This may imply directwriting in a write-mechanism-plus-medium combination. Alternatively, themedium may be realized by copying a master encoded medium such as astamp. In block 32, the channel bits are read again from the medium,followed by immediate demodulating. This will produce recognized syncpatterns, as well as code symbols that must be further decoded. Nowgenerally, sync patterns occur on positions where a player device wouldindeed expect them, leading to the conclusion that synchronization iscorrect. However, correct sync patterns may be found on unexpectedpositions. This may indicate loss of synchronism, which must be restoredin a tedious process that bases itself on various successive syncpatterns received. Generally, synchronization is maintained through aflywheel procedure, or as based on a majority decision among a pluralityof successive sync patterns. A correct sync pattern may alternatively befound due to one or more channel bit errors in the data on otherpositions than intended for sync patterns. Generally, such will be anisolated feature and not lead to a resynchronizing effort. On the otherhand, on an expected position the demodulator may fail to findsynchronizing pattern. Often, the error will be a random bit that isrestorable through inherent redundancy in the synchronizing pattern.This will then lead to a correct sync pattern, and allow to proceed in astandard manner without further considering the restored synchronizingpattern for other channel bits. Alternatively, the error is sufficientlyserious as to lead to the conclusion that a burst has been encountered.Such burst may then provide a clue for signalling other symbols in thephysical neighbourhood thereof as being erroneous, in the same manner aswill hereinafter be discussed with respect to clue words. In principle,the sync-derived clues could be sufficient for enhancing the standarderror protectivity. For this purpose, they should be located not too farfrom each other. If clues are derived from synchronizing patterns aswell as from clue words, the synchronizing patterns may be used as aseparate mechanism for generating clues even before the start of thedecoding of the clue words. This might allow to use two clue mechanismsside by side, one from the synchronizing patterns and one from the cluewords. Alternatively, the clues from the synchronizing patterns and fromthe clue words may be combined. The selecting among the variousmechanisms so recited may be done on the basis of a static or dynamicpolicy. Quite often, the combining with clues found through the decodingof clue words may be used to get a better or more powerful decoding oftarget words.

After demodulating, the clue words are sent to decoder 34, and decodedas based on their inherent redundancies. As will become apparent in thediscussion of FIG. 3 infra, such decoding may present clues on thelocations of errors in other than these clue words. Box 35 receivesthese clues and as the case may be, other indications via arrow 33, andoperates on the basis of a stored program for using one or moredifferent strategies to translate clues into erasure locations or otherindications for identifying unreliable symbols. The input on line 33 mayrepresent clues produced by the demodulating of the sync bit groups, oras the case may be, other indications such as produced by the generalquality of the received signal, such as derived from its frequencyspectrum. The target words are decoded in decoder 36. With help from theerasure locations or other identifications, the error protectivity ofthe target words is raised to a higher level. Finally, all decoded wordsare multiplexed by means of element 38 conformingly to the originalformat to output 40. For brevity, mechanical interfacing among thevarious subsystems has been ignored.

FIGS. 2B, 2C illustrate further dispositions of exemplary sync patterns,as distributed in the information stream. Each individual sync patternmay look like FIG. 2A. In the first place, these sync patterns may bethe only source for the clue information. They are preferably positionedin periodically spaced locations in the information stream.Alternatively, clues may derive both from the sync patterns and from theclue words. FIGS. 2B, 2C illustrate the latter case. Therein, thepositions of the clue word symbols have been indicated by crosses. Thepositions of the sync bit groups have been indicated by dots. In FIG.2B, 2C, distances between clue word symbols are higher at the locationof the sync bit group shown than elsewhere, so that they are locallyscarcer. In FIG. 2A their distance is less than twice its valueelsewhere. In FIG. 2B it is equal to twice its value elsewhere. Otherdistributions are feasible.

FIG. 3 illustrates a simple code format without contribution by the syncbit groups. The coded information of 512 symbols has been notionallyarranged in a block of 16 rows and 32 columns. On-medium storage isserially column-by-column starting at the top left. The hatched regioncontains check symbols, and clue words 0, 4, 8, and 12 have 8 checksymbols each. The target words contain 4 check symbols each. The wholeblock contains 432 information symbols and 80 check symbols. The lattermay be localized in a more distributed manner over their respectivewords. A part of the information symbols may be dummy symbols. TheReed-Solomon code allows to correct in each clue word up to four symbolerrors. Actually present symbol errors have been indicated by crosses.In consequence, all clue words may be decoded correctly, inasmuch asthey never have more than four errors. Notably words 2 and 3 may howevernot be decoded on the basis of their own redundant symbols only. Now, inFIG. 3 all errors, except 62, 66, 68 represent error strings, but onlystrings 52 and 58 cross at least three consecutive clue words. These areconsidered as error bursts and cause erasure flags in all intermediatesymbol locations. One or more target words before the first clue worderror of the burst and one or more target words just after the last cluesymbol of the burst may also get an erasure flag, depending on strategy.String 54 is too short to be considered a burst.

Therefore, two of the errors in word 4 produce an erasure flag in theassociated columns. This renders words 2 and 3 correctable, each with asingle error symbol and two erasure symbols. However, random errors 62,68, nor string 54 constitute clues for words 5, 6, 7, because each ofthem contains only a single clue word. Sometimes, an erasure may resultin a zero error pattern, because an arbitrary error in an 8-bit symbolhas a 1/256 probability to cause a correct symbol. Likewise, a burstcrossing a particular clue word may produce a correct symbol therein. Abridging strategy between preceding and succeeding clue symbols of asingle burst may incorporate this correct symbol into the burst, and inthe same manner as erroneous clue symbols may translate it into erasuresfor appropriate target symbols.

DISCUSSION OF A PRACTICAL FORMAT

Practising the invention applies to new methods for digital opticalstorage. At present, substrate incident reading may have a transmissivelayer as thin as 100 micron. Channel bits may have a size of some 0.14microns, so a data byte at a channel rate of 2/3 will have a length ofonly 1.7 microns. At the top surface the beam has a diameter of 125microns. Enveloping the disc in a so-called caddy will reduce theprobability of large bursts. However, non-conforming particles of lessthan 50 microns may cause short faults. Developers have used a faultmodel wherein such faults through error propagation may lead to burstsof 200 microns, corresponding to some 120 Bytes. The model proposesfixed size bursts of 120 B that start randomly with a probability perbyte of 2.6*10⁻⁵, or on the average one burst per 32 kB block. Theinventor envisages serial storage on optical disc, but configurationssuch as multitrack tape, and other technologies such as magnetic andmagneto-optical would benefit from the improved approach herein.

FIG. 4 shows a picket code and burst indicator subcode. A picket codeconsists of two subcodes A and B. The Burst Indicator Subcode containsthe clue words. It is formatted as a deeply interleaved long distancecode that allows to localize the positions of the multiple burst errors.The error patterns so found are processed to obtain erasure informationfor the target words that are configured in the embodiment as a ProductSubcode. The latter will correct combinations of multiple bursts andrandom errors, by using erasure flags obtained from the burst indicatorsubcode. The indicators provided by the sync bit groups may be used inisolation, or in combination with the indications from the clue words.Generally, if no due words had been provided at all, the number ofsynchronizing bit groups will be raised. The developing of the clueswill be similar to the procedure sketched with reference to FIG. 3 forthe clue words.

The following format is proposed:

the block of 32 kB contains 16 DVD-compatible sectors

each such sector contains 2064=2048+16 Bytes data

each sector after ECC encoding contains 2368 Bytes

therefore, the coding rate is 0.872

in the block, 256 sync blocks are formatted as follows

each sector contains 16 sync blocks

each sync block consists of 4 groups of 37 B

each group of 37 B contains 1 B of deeply interleaved Burst IndicatorSubcode and 36 B of Product Subcode.

In FIG. 4, rows are read sequentially, each starting with its leadingsync pattern. Each row contains 3 Bytes of the BIS shown in grey,numbered consecutively, and spaced by 36 other Bytes. Sixteen rows formone sector and 256 rows form one sync block. Overall redundancy has beenhatched. The synchronization bytes may be used to yield clues, throughredundancy therein outside the main code facilities. The hardwarearrangement of FIG. 1 may execute the processing of the synchronizationbit groups that constitute words of another format than the data bytesin a preliminary operation step. Still further information may indicatecertain words or symbols as unreliable, such as from the quality of thesignal derived from the disc or through demodulation errors. Note thatin FIG. 4 one column at left is now no longer necessary for the cluewords of the burst indicator subcode BIS. As shown, this column iffilled with target words. Alternatively, the column is left outcompletely. In both cases, the next storage density for user data isincreased.

The sync bit group is a good vehicle for detecting bursts through itsinherently large Hamming distance from most burst-inflicted patterns. Atypical spacing between sync bit groups could be about 1000 channelbits. A different format is to split a 24-bit sync bit pattern into twohalves of twelve bits each, that each violate the modulation principleonly once. The spacing between sync bit groups is then also halved toabout 500 bits, so that overhead remains the same. It is possible toexclusively use predetermined bit fractions from the sync bit group forburst detection. Note that in FIG. 3, the sync bit group would occupy ahorizontal row above the first clue word position.

What is claimed is:
 1. A method for encoding multiword information, comprising: providing multibit information words; splitting the multibit information words into first multibit information words and second multibit information words; encoding the first multibit information words into first code words of a first multi-symbol error correcting code, said first code words including error protected clue words, said clue words providing first clues for locating errors in said multibit information words; and encoding the second multibit information words into second code words of a second multi-symbol error correcting code, said second code words including target words, said clue words having a higher degree of error protection than said target words.
 2. The method of claim 1, further comprising: modulating the first and second code words into channel bit groups in accordance with a constraint; and writing the channel bit groups onto a unitary medium, said channel bit groups on said unitary medium comprising synchronizing bit patterns.
 3. The method of claim 2, wherein the synchronizing bit patterns comprise a first synchronizing bit pattern, a second synchronizing bit pattern, and a third synchronizing bit pattern, wherein a first clue word relating to the error protected clue words is disposed between the first and second synchronizing bit patterns, wherein no clue word relating to the error protected clue words is disposed between the second and third synchronizing bit patterns, and wherein the distance between the first and second synchronizing bit patterns exceeds the distance between the second and third synchronizing bit patterns.
 4. The method of claim 2, wherein a first channel bit group of the channel bit groups violates the constraint, and wherein a second channel bit group of the channel bit groups does not violate the constraint.
 5. The method of claim 1, wherein the first code and the second code are a same code.
 6. The method of claim 1, wherein the first code and the second code are different codes.
 7. The method of claim 1, wherein the first code is selected from the group consisting of a Reed-Solomon code, a product code, and an interleaved code.
 8. The method of claim 1, wherein the second code is selected from the group consisting of a Reed-Solomon code, a product code, and an interleaved code.
 9. The method of claim 1, wherein the first code and the second code are a Reed-Solomon code, and wherein the first code is a subset of the second codes or the second code is a subset of the first code.
 10. The method of claim 1, wherein the first code words have a length, and wherein the second code words have the length.
 11. The method of claim 1, wherein the first code words have a first length, wherein the second code words have a second length, and wherein the first length differs from the second length.
 12. A method for decoding multiword information, comprising: providing a unitary medium comprising data embedded therein, said data including channel bit groups comprising synchronizing bit patterns, wherein the synchronizing bit patterns provide first clues for locating errors in the data, reading the data from the unitary medium to form an information stream that comprises the channel bit groups; demodulating the channel bit groups to generate demodulated information words; and deriving clue words from the demodulated information words, wherein the clue words provide second clues for locating errors in the data.
 13. The method of claim 12, further comprising using said first and second clues in a cooperative manner to locate said errors in the data.
 14. The method of claim 12, further comprising using the second clues to locate said errors in the data.
 15. The method of claim 12, further comprising determining whether to use the first clues, the second clues, or the first and second clues to locate said errors in the data, said determining being based on a static policy.
 16. The method of claim 12, further comprising determining whether to use the first clues, the second clues, or the first and second clues to locate said errors in the data, said determining being based on a dynamic policy.
 17. The method of claim 12, further comprising decoding the clue words to find the second clues.
 18. A method for decoding multiword information, comprising: providing a unitary medium comprising data embedded therein, said data including channel bit groups comprising synchronizing bit patterns, wherein the synchronizing bit patterns provide first clues for locating errors in the data, wherein the channel bit groups further comprise clue words of bits, wherein the clue words provide second clues for locating errors in the data, wherein the synchronizing bit patterns comprise a first synchronizing bit pattern, a second synchronizing bit pattern, and a third synchronizing bit pattern, wherein a first clue word of the clue words is disposed between the first and second synchronizing bit patterns, wherein none of said clue words is disposed between the second and third synchronizing bit patterns, and wherein the distance between the first and second synchronizing bit patterns exceeds the distance between the second and third synchronizing bit patterns.
 19. A device for encoding multiword information, comprising: splitting means for splitting multibit information words into first multibit information words and second multibit information words; first encoding means for encoding the first multibit information words into first code words of a first multi-symbol error correcting code, said first code words including error protected clue words, said clue words providing first clues for locating errors in said multibit information words; and second encoding means for encoding the second multibit information words into second code words of a second multi-symbol error correcting code, said second code words including target words, said clue words having a higher degree of error protection than said target words.
 20. The device of claim 19, further comprising: modulating means for modulating the first and second code words into channel bit groups in accordance with a constraint; and writing means for writing the channel bit groups onto a unitary medium, said channel bit groups on said unitary medium comprising synchronizing bit patterns.
 21. The device of claim 20, wherein the synchronizing bit patterns comprise a first synchronizing bit pattern, a second synchronizing bit pattern, and a third synchronizing bit pattern, wherein a first clue word relating to the error protected clue words is disposed between the first and second synchronizing bit patterns, wherein no clue word relating to the error protected clue words is disposed between the second and third synchronizing bit patterns, and wherein the distance between the first and second synchronizing bit patterns exceeds the distance between the second and third synchronizing bit patterns.
 22. The device of claim 20, wherein a first channel bit group of the channel bit groups violates the constraint, and wherein a second channel bit group of the channel bit groups does not violate the constraint.
 23. A device for decoding multiword information words, comprising: reading means for reading data from a unitary medium to form an information stream, wherein the data comprises channel bit groups having synchronizing bit patterns, wherein the synchronizing bit patterns provide first clues for locating errors in the data; demodulating means for demodulating the channel bit groups to generate demodulated information words; and deriving means for deriving clue words from the demodulated information words, wherein the clue words provide second clues for locating errors in the data.
 24. The device of claim 23, further comprising means for using said first clues or said second clues to locate said errors in the data.
 25. The device of claim 23, further comprising means for using said first and second clues in a cooperative manner to locate said errors in the data.
 26. The device of claim 23, wherein the channel bit groups further comprise clue words of bits, wherein the clue words provide second clues for locating errors in the data, wherein the synchronizing bit patterns comprise a first synchronizing bit pattern, a second synchronizing bit pattern, and a third synchronizing bit pattern, wherein a first clue word of the clue words is disposed between the first and second synchronizing bit patterns, wherein none of said clue words is disposed between the second and third synchronizing bit patterns, and wherein the distance between the first and second synchronizing bit patterns exceeds the distance between the second and third synchronizing bit patterns.
 27. A carrier comprising: a data-readable medium having data embedded therein, said data including channel bit groups comprising synchronizing bit patterns, synchronizing bit patterns providing first clues for locating errors in the data, said data adapted to be read from the medium to form an information stream that comprises the channel bit groups, wherein the channel bit groups further comprise clue words of bits, and wherein the clue words provide second clues for locating errors in the data.
 28. The carrier of claim 27, wherein the synchronizing bit patterns comprise a first synchronizing bit pattern, a second synchronizing bit pattern, and a third synchronizing bit pattern, wherein a first clue word of the clue words is disposed between the first and second synchronizing bit patterns, wherein none of said clue words is disposed between the second and third synchronizing bit patterns, and wherein the distance between the first and second synchronizing bit patterns exceeds the distance between the second and third synchronizing bit patterns.
 29. The carrier of claim 27, wherein the channel bit groups were formed from modulation of code words in accordance with a constraint, wherein a first channel bit group of the channel bit groups violates the constraint, and wherein a second channel bit group of the channel bit groups does not violate the constraint. 